Event Description:

Title: Explosions in the Sky: Computational Modeling of Asteroid Airbursts

Abstract

Computational models of asteroid airbursts on Earth are rooted in simulations of the 1994 impact of Comet Shoemaker-Levy 9 on Jupiter. The 1908 Tunguska explosion in Siberia is the best-known example. Models suggest that the altitude of maximum energy deposition is not a good estimate of the equivalent height of a point explosion. The 2013 Chelyabinsk event yielded direct observational data that could be compared to models, confirming this conclusion. The center of mass of an exploding projectile maintains a significant fraction of its initial momentum and is transported downward and forward. The fireball—a hot jet of ablated asteroid—descends to a depth well beneath the burst altitude before its velocity becomes subsonic. Stronger blast waves and thermal radiation pulses are therefore experienced at the surface than for an equivalent nuclear explosion. In the Tunguska case, the jet lost its momentum before making contact with the Earth's surface. For impacts above some energy threshold, the fireball reaches the ground, expands radially, and drives supersonic hot winds that can melt surface materials. The Libyan Desert Glass event (~29 million years ago) may be an example of this second, larger and more destructive type of airburst. Recent research has suggested that such airbursts can generate tsunami waves, but the efficiency of this coupling remains controversial. Better understanding of airbursts, combined with the diminishing number of undiscovered large asteroids, leads to the conclusion that airbursts represent a large and growing fraction of the total impact threat. The most effective policies to reduce the impact threat would therefore be to raise awareness of tsunami risk even in seismically inactive ocean basins, and to resurrect the old cold-war “duck-and-cover” concept to minimize casualties from air blast effects.

The author acknowledges Sandia National Laboratories, DOE, NASA, FEMA, and DTRA for funding various aspects of this research.

Biosketch: Mark Boslough

Mark Boslough received his Ph.D. in Applied Physics from Caltech and joined Sandia National Laboratories in 1983, where he has worked on many aspects of impact physics including NMR spectroscopy of shocked sandstone, testing space-station debris shields, and analyzing satellite observations of fireballs. In 1994 he was a member of a team that gained international recognition for using a supercomputer to correctly predict the effects of the impact of Comet Shoemaker-Levy 9 on Jupiter. His current impact research is focused on computational modeling of airbursts and their effects. He participated in expeditions to airburst sites in the Libyan Desert of Egypt in 2006, and to Tunguska in Siberia in 2008. His has collaborated on research to understand Asteroid 2008 TC3, its spectacular airburst over northern Sudan, the recovery of its meteorites, and its implications for understanding the impact threat. He provided modeling support to explain the phenomena associated with the 2009 and 2010 impacts on Jupiter. He served on the asteroid mitigation panel for the National Research Council, and coauthored the report “Defending Planet Earth” that was delivered to Congress in 2010. In 2013 he was the first US scientist to visit the site of the Chelyabinsk airburst in Russia to begin the process of documentation, appearing in two episodes of PBS NOVA. His simulation of that event appeared on the cover of Nature in November, 2013 and Physics Today in September, 2013. He is currently working on many aspects of planetary defense, including the contribution of impact-generated tsunami to overall impact risk.